The present invention relates to:
a glass-ceramic, which is opaque, and has a low, or zero thermal expansion coefficient, and which contains a solid solution of xcex2-quartz as crystalline phase; and
a process of obtaining said glass-ceramic.
Glass-ceramics are materials which are known per se, and which are obtained by heat treatment of mineral glasses.
These materials are used in various contexts and notably as substrates of cooktops and as anti-fire windows (having a resistance to thermal breakage and thus preventing the spread of a fire).
It is important for the applications of this nature that said glass-ceramics resist not only to the high temperatures but also to the wide variations in temperature. In particular, in the case of a substrate for cooktops, such variations in temperature manifest themselves not only rapidly but also in a non-uniform manner within the mass, such that a part of said substrate can cool down or heat up very rapidly, while the temperature of the other parts can remain unchanged or can vary much more slowly. Thus, it is desirable to have glass-ceramics at one""s disposal which have a very low thermal expansion coefficient, preferably with such a thermal expansion coefficient of zero or near-zero.
Furthermore, transparent and opalescent glass-ceramics, even opaque glass-ceramics are known.
More specifically:
the article by G. H. Beall entitled: xe2x80x9cCrystallization and Chemical Strengthening of Stuffed xcex2-Quartz Glass-Ceramicsxe2x80x9d, in the Journal of the American Ceramic Society, Vol. 50., No. 4., p. 181-190 (April 1967) gives, in general, xcex2-quartz glass-ceramics (which contain a solid solution of xcex2-quartz as crystalline phase). Said article indicates the ranges of composition of glass which enable said glass-ceramics to be obtained, the ranges of ceramming temperature, the ranges of expansion of said glass-ceramics, the possibility of high-temperature decomposition of the xcex2-quartz phase. Said article describes no opaque glass-ceramic of zero expansion and all the time indicates ceramming times of at least 4 hours;
the patent application EP-A-0-853 071 describes xcex2-quartz and xcex2-spodumene glass-ceramics (which contain a solid solution of xcex2-quartz or xcex2-spodumene, respectively, as crystalline phase). The described xcex2-quartz glass-ceramics (which are not opaque) are different from the ones of the invention;
the patent application EP-A-1 029 830 describes xcex2-quartz glass-ceramics (which contain a solid solution of xcex2-quartz as crystalline phase), which are opaque, and which are of zero expansion. However, said glass-ceramics are different from the glass-ceramics of the invention and the ceramming times described are of the order of at least 6 hours;
U.S. Pat. No. 5,070,045 describes xcex2-quartz glass-ceramics (which contain a solid solution of xcex2-quartz as crystalline phase), which have zero expansion, which are transparent, and obtainable by a ceramming treatment of less than 2 hours. The inventors wished to confer an opaque aspect to these glass-ceramics. Thus, they crystallised the glass precursors of said glass-ceramics at a higher temperature (around  greater than 950xc2x0 C.). However, the opacification is then accompanied by a change of crystalline phase: the main phase becomes a solid solution of xcex2-spodumene. This conversion has the drawback, for the applications sought after, of being accompanied with an increase in the thermal expansion coefficient. This latter value becomes close to 10xc3x9710xe2x88x927 Kxe2x88x921.
The glass compositions described in U.S. Pat. No. 5,070,045 thus have been modified, in accordance with the invention, in order to be able to generate opaque glass-ceramics of zero expansion. It was sought to obtain the opacification while at the same time preventing the conversion, of the xcex2-quartz phase into the xcex2-spodumene phase, which leads to the increase of the expansion. The conversion into other phases of high expansion (such as spinel) should also be avoided.
It is noted at this juncture that the presence of a solid solution of xcex2-quartz, as main phase, is not in itself a guarantee to obtain a zero expansion. As indicated in the article cited above, numerous oxides can enter into the composition of this solid solution, in particular Li2O, MgO, ZnO and Al2O3. According to the content of each one of these oxides, the expansion of the crystalline phase and, thus, that of the glass-ceramic, can vary within a very large range (of around xe2x88x9220 to +50 10xe2x88x927Kxe2x88x921 for the glass-ceramic).
The technical problem tackled has thus been that of determining the levels of the various oxides not only in order to avoid the conversion into xcex2-spodumene, but more specifically to obtain, advantageously in less than two hours, an opaque glass-ceramic with a thermal expansion coefficient of practically zero.
It was observed in a surprising way that this result could be obtained, with respect to the teaching of U.S. Pat. No. 5,070,045, by lowering the content of Li2O, while at the same time limiting the content of MgO and by increasing the content of ZnO; contents of Li2O, MgO and ZnO of the precursor mineral glasses and of the final glass-ceramics.
The glass-ceramics of the invention are thus opaque glass-ceramics which are particularly interesting with reference to their thermal expansion coefficient and which moreover have the advantage of being able to be obtained at the end of a short heat treatment (of less than two hours) of their precursor glasses.
According to a first object, the present invention relates to original glass-ceramics which:
are opaque;
have a low, or zero thermal expansion coefficient (of 0xc2x15xc3x9710xe2x88x927 Kxe2x88x921, advantageously of 0xc2x13xc3x9710xe2x88x927 Kxe2x88x921);
contain a solid solution of xcex2-quartz as crystalline phase, and
which composition essentially consists, by weight percent, of:
65 to 72% of SiO2 
16 to 24% of Al2O3 
2 to less than 3% of Li2O,
0 to 1% of MgO
3 to 5% of ZnO
0 to 1.5% of BaO
2 to 4.5% of TiO2 and/or ZrO2, with the Al2O3/(TiO2+ZrO2)ratio  greater than 5, 0 to an effective and non excessive amount of a least one fining agent, and less than 1% of K2O and/or Na2O.
From mineral glasses having the composition indicated above (for the glass-ceramics of the invention obtained from said mineral glasses) the inventors have surprisingly obtained opaque glass-ceramics which have low or zero thermal expansion coefficient and which contain a solid solution of xcex2-quartz as crystalline phase.
In reference to the said composition (of the glass-ceramics of the invention as well as of the mineral glasses, precursors of the said glass-ceramics of the invention) the following may be precised.
SiO2 is a classical component which forms mineral glasses. Its content must be less than 72% in order to limit the melting point and greater than 65% in order to avoid devitrification during cooling.
Al2O3 is incorporated at a relatively significant content since it is one of the main constituents of the crystalline phase. Surprisingly, it was observed that it was easier to obtain an opaque material when said Al2O3 content increased. With too high an Al2O3 content, the tendency of devitrification during cooling becomes unacceptable.
Li2O is a key element of the crystallisation. When it is incorporated at a content of less than 2%, it is difficult to obtain the crystalline phase. When it is incorporated at a content of greater than 3%, the opacification tends to be accompanied by a conversion into xcex2-spodumene.
MgO tends to increase the expansion: it is for this reason that its content is limited to 1%. Its presence is not indispensable.
K2O and Na2O also tends to increase the expansion: it is for this reason that their content (K2O alone, Na2O alone or K2O+Na2O in combination) is limited to 1%, advantageously 0.5% (the composition advantageously comprises less than 0.5% of K2O+Na2O). Their presence is in no way indispensable, is actually undesirable. Said elements are advantageously absent and are actually generally present only as impurities.
ZnO is in quite a significant amount in order to compensate the low levels of Li2O and MgO.
TiO2 and ZrO2 are the nucleation agents. They can be incorporated independently of each other (TiO2 or ZrO2) or, advantageously, in combination (TiO2 and ZrO2). In order to facilitate obtaining opaque materials, their total content must not be greater than 4.5%. In order to maintain a satisfactory opacity, it was observed that if the Al2O3 content decreases, the total content of nucleation agents should decrease. It is for this reason that it is highly desirable that the Al2O3/(TiO2+ZrO2) ratio remain greater than 5.
BaO is an optional component. It enables the viscosity of the glass to be lowered and from this, it facilitates the melting and the ceramming.
Advantageously, at least one fining agent is incorporated in an effective and non-excessive amount. Such an effective and non-excessive amount is generally inferior or equal to 2%. As2O3 and/or Sb2O3 may be used as classical fining agent. The use of other fining agents such as SnO2, CeO2 and Cl is not excluded.
As well as the essential and optional constituents set forth above, the glass-ceramics of the invention can contain other constituents. Obviously, they contain such other constituents only in a limited amount (generally less than 5%, more generally less than 2%), only an amount which does not jeopardise the characteristics of the glass-ceramics of the invention. Thus, it is in no way excluded that the glass-ceramics of the invention contain CaO, SrO, SnO2 and/or P2O5.
Preferably, the glass-ceramics of the invention essentially have the composition below, expressed in percentages by weight of oxides:
65 to 72% of SiO2 
17 to 21% of Al2O3 
2 to less than 3% of Li2O
0 to 1% of MgO
3 to 4.5% of ZnO
0 to 1.5% of BaO
1.5 to 3% of TiO2 
0.5 to 1.8% of ZrO2 
with TiO2+ZrO2xe2x89xa64.5%, and
the Al2O3/(TiO2+ZrO2) ratio  greater than 5
0 to an effective and non excessive amount of at least one fining agent, and
less than 1% (advantageously 0.5%) of K2O and/or Na2O.
The preferred ranges indicated above can be considered independently of each other. Taken in combination, they define a preferred composition by weight of the glass-ceramics of the invention.
In general, the said glass-ceramics of the invention advantageously have an MgO content of between 0 and 0.5%.
According to a second object, the present invention relates to a process for obtaining the glass-ceramics of the invention. The said process classically comprises:
preparing, in the molten state, a mineral glass, precursor of a glass-ceramic;
shaping said molten mineral glass; and
heat treating said shaped molten mineral glass.
Characteristically, in order to obtain the said glass-ceramics of the invention, the precursor mineral glass has a specific composition and the heat treatment is carried out at a relatively high temperature.
The said process of the invention actually comprises the following steps:
preparing, in the molten state, a mineral glass, which composition essentially consists, by weight percent, of:
65 to 72% of SiO2 
16 to 24% of Al2O3 
2 to less than 3% of Li2O,
0 to 1%of MgO
3 to 5% of ZnO
0 to 1.5% of BaO
2 to 4.5% of TiO2 and/or ZrO2, with the Al2O3/(TiO2+ZrO2)ratio  greater than 5,
0 to an effective and non excessive amount of a least one fining agent, and
less than 1% of K2O and/or Na2O;
shaping said molten mineral glass; and
heat treating said shaped molten mineral glass at a ceramming temperature of at least 980xc2x0 C., generally at a temperature between 980xc2x0 C. and 1 150xc2x0 C.
Advantageously, the precursor mineral glass has essentially the following composition, expressed in weight percent:
65 to 72% of SiO2 
17 to 21% of Al2O3 
2 to less than 3% of Li2O
0 to 1% of Mgo
3 to 4.5% of ZnO
0 to 1.5% of BaO
1.5 to 3% of TiO2 
0.5 to 1.8% of ZrO2 
with TiO2+ZrO2xe2x89xa64.5%, and
the Al2O3/(TiO2+ZrO2) ratio  greater than 5
0 to an effective and non excessive amount of at least one fining agent, and
less than 1% (advantageously 0.5%) of K2O and/or Na2O.
The ceramming heat treatment is carried out at a temperature of at least 980xc2x0 C. It is generally carried out at a temperature between 980xc2x0 C. and 1 150xc2x0 C.; it is advantageously carried out at a temperature between 1 000 and 1 125xc2x0 C.
It is quite possible and interesting to obtain the glass-ceramics of the invention with a duration of the ceramming heat treatment (also called ceramming time) not exceeding 2 hours.
It is not excluded from the context of the present invention to carry out ceramming for more than 2 hours. However, at the above mentioned high ceramming temperatures, it may be possible to convert the xcex2-quartz phase into a xcex2-spodumene one and to increase the thermal expansion coefficient. The man skilled in the art will be able to determine and optimise the process parameters (temperature, duration, composition of the mineral glass) to obtain the opaque expected glass-ceramics.
According to an embodiment, the heat treatment in question includes a heating up to the temperature, known as the ceramming temperature, followed by an upkeep of said ceramming temperature. Said upkeep can be limited to 15 minutes. Said ceramming temperature is of at least 980xc2x0 C., generally between 980xc2x0 C. and 1 150xc2x0 C., advantageously between 1 000 and 1 125xc2x0 C. (see above).
Furthermore, it was observed that the thermal expansion coefficient of the glass-ceramics of the invention varies with their ceramming temperature, i.e. the temperature at which their glass precursor was brought to and kept at; said coefficient increases with said ceramming temperature. Thus, it is possible to determine, on the glass samples, the ceramming temperature which leads to a glass-ceramic having a given low thermal expansion coefficient, which is advantageously zero, and then to carry out the production of said glass-ceramic by ensuring the ceramming with an upkeep of the ceramming temperature determined beforehand.
Thus, the implementation of the process for obtaining a glass-ceramic of the invention is advantageously preceded by a prior experimentation on the glass concerned, with the view to detecting the ceramming temperature of said glass which generates a glass-ceramic having a thermal expansion coefficient which is fixed, low, advantageously zero; said implementation thus comprises, within the context of the heat treatment, the upkeep of the glass at said ceramming temperature determined beforehand.